During a typical process of fingerprint identification, a portion (i.e. a detecting panel) of a fingerprint sensor contacts with a finger. The detecting panel includes a plurality of small detecting units and one detecting unit can be called one detecting pixel, in which these detecting pixels can be distributed two-dimensionally. When fingerprinting, the finger is placed on the detecting panel. Because of the shape of the finger, a peak of the pattern is directly contacted with the detecting panel but a valley of the pattern is at a short distance from the detecting panel, in which the short distance is corresponding to a depth of the valley. If the short distance can be identified, the peak and the valley of the pattern can be distinguished, i.e. a fingerprint characteristic can be detected. Another way is to change an electric field distribution on a surface of the detecting panel by the finger, and further to detect the fingerprint characteristic.
The above-mentioned fingerprint detecting method may be implemented by a capacitance detecting method. The fingerprint detecting method based on this principle already has a variety of implementations. One of the implementations is to detect a capacitance formed between the finger and a top metal plate below the detecting panel to determine the fingerprint characteristic. Specifically, in the structure, one detecting pixel corresponds to one metal electrode and those metal electrodes are two-dimensionally distributed in the directions of a row and a column to constitute a two-dimensional detecting array as one plate for fingerprint detecting. At the same time, the finger placed on the detecting panel constitutes another plate corresponding to the two-dimensional detecting array due to a conductive characteristic of the finger, so a capacitance formed between the two plates is corresponding to the fingerprint characteristic. If the distance between the two plates corresponding to the peak is small then the capacitance is large, and vice versa for the valley. Therefore, the fingerprint characteristic can be determined by detecting the capacitance.
Another of the implementations is as follows: each detecting pixel includes two plates, and these plates are alternately arranged and located on different metal layers, and a fixed capacitance can be formed between these plates. The electric field distribution between these plates can be changed when the finger is placed on the detecting panel, and then the fixed capacitance also can be changed, so the peak and valley of the pattern can be distinguished, i.e. the fingerprint characteristic can be detected due to the different electric field distribution of the peak and valley.
With the fingerprint sensor implemented with a semiconductor sensing capacitor, a superstructure thereof facing the finger typically includes multiple conductive layers of metal and isolative layers between the conductive layers. The uppermost conductor layer is the fingerprint detecting plate corresponding to the finger. Capacitances formed between the conductive layers can be used as an integrating capacitance which is used for detecting fingerprint and formed between the finger and the uppermost conductor layer or can be used as parasitic capacitances. However, the accuracy of the fingerprint detecting can be affected by parasitic capacitances.
However, parasitic capacitances not only exist between the conductive layers, but also exist between detecting electrodes of the uppermost conductor layers, and between conductive layers and the ground. The fact that parasitic capacitances influence the accuracy of the fingerprint detecting manifests in two aspects: on the one hand, usually parasitic capacitances are parallel with the detecting capacitance, for example, an input signal is changed from Vin to Vin*Cf(Cf+Cp), where Vin represents the input voltage signal, Cf represents the detecting capacitance formed by the finger, and Cp represents parasitic capacitances, and parasitic capacitances Cp is larger than the detecting capacitance Cf, which significantly reduces the accuracy of the fingerprint detecting; on the other hand, a noise from the substrate is usually coupled to the detecting capacitance by parasitic capacitances, which further reduces the accuracy of the fingerprint detecting.